When considering understanding the contact state of a contact surface using a tactile sensor, there are vectors of three components representing magnitude and direction of force acting at each point of the contact surface. This is represented as f(x,y) in the coordinate system of FIG. 1. Here, f is a vector, and so actually has three components x, y and z at each point. When explicitly expressing each component, it is represented as f(x,y)=[fx(x,y), fy(x,y), fz(x,y)]. Since force distribution has three components at each contact point, in order to reconstruct force distribution for each contact surface using a tactile sensor, it is necessary to acquire information for each contact point on the contact surface with at least three degrees of freedom.
Some of inventors of the present invention et al. have proposed an optical tactile sensor that is capable of measuring three-dimensional force vector distribution. A principle of the optical tactile sensor will be explained based on FIG. 2. The optical tactile sensor comprises a transparent elastic body and a CCD camera. By photographing spherical markers embedded in the transparent elastic body by the CCD camera, internal strain information of the elastic body is measured when a force is applied on the surface of the elastic body, and force vector distribution is reconstructed from the information.
By taking an image of the spherical markers by a CCD camera from z-direction where an elastic body surface is taken as the x-y plane and an orthogonal direction to the x-y plane is taken as the z-axis, movement of a point to be measured when force is applied is measured as a movement vector in the x-y plane. To reconstruct the force vector distribution from the strain information, N×N red spherical markers and blue spherical markers are arranged at different depths in the elastic body as points to be measured to obtain two sets of two-dimensional movement vectors with different depths as two pieces of different information, thereby increasing the amount of information to reconstruct the force vector distribution.
As a use for such an optical tactile sensor, initially, application to a robotic hand of a humanoid robot and so forth is considered, and as an application for an optical tactile sensor, study has focused on a small built-in type sensor. However, such an optical tactile sensor, which is capable of measuring three dimensional force vector distribution and has a sensor surface made of a elastic body, is expected to find application in a variety of fields, not only to a robotic hand.
As one of the expected applications, use of an optical tactile sensor as so called an environmental type tactile sensor can be considered. In this specification, as compared to a built-in type tactile sensor which is incorporated for use into a robotic hand or the like, a tactile sensor which is used in a fixed manner in an environment is referred to as an environmental type tactile sensor. However, with respect to an environmental type tactile sensor, which is used in a fixed manner in an environment, measurement of force distribution over a large area is expected to be necessary because such a sensor is assumed to be applied to, for example, a seating-surface of a chair, a bed, a floor, or the like. This hinders application of a conventional small optical tactile sensor, which is assumed to be incorporated in a robot hand or the like
Patent Reference: WO02/188923 A1
An object of the present invention is to provide an optical tactile sensor capable of measuring force distribution over a large area.
Another object of the present invention is to provide a marker information acquisition method and a marker image integration method capable of measuring force distribution over a large area. Still another object of the present invention is to provide a force vector reconstruction method employed in an optical tactile sensor capable of measuring force distribution over a large area.